Look at it another way: if people like your book enough to go to the effort of making copies and giving them away to their friends, this is just increasing the likely audience for your next book! A word-of-mouth promotional campaign they are doing for you.
Another thought, p2p wise, is to release and promote the book on the p2p network yourself! Share it on a torrent site, with a note that this is YOUR work, and if people like it, to please send a donation your way or link the sale site for the ebook so they have the option of rewarding you for your work. I know this has worked for some small bands, and people do like supporting something that they know comes directly from the artist - heck, some torrent site moderators may even give you a little front page promotion and back the idea of donating.